Coated metal mold

ABSTRACT

A METAL MOLD HAVING A MOLD CAVITY FOR CASTING METALS THEREIN, THE WALL OF WHICH COMPRISES A SUBSTRATE MADE OF CAST IRON OR MILD STEEL, AN INTERMEDIATE LAYER ABOUT 30 TO ABOUT 100 MICRONS THICK FORMED ON THE SURFACE OF SAID SUBSTRATE AND CONTAINING ABOUT 6 TO ABOUT 12 PERCENT BY WEIGHT OF SILICON DIFFUSED THEREIN AND A LAYER TO NICKEL-ALUMINUM OR NICKEL-CHROMIUM ALLOY DEPOSITED ON THE SURFACE OF SAID INTERMEDIATE LAYER BY MEANS OF PLASMA SPRAYING OR FLAME SPRAYING.

United States Patent 1,699,612 7 1/1929 Doat Herurniclil Okamoto;

Toshlo Shilrata; Kazuo Magaribuchi, Yokohama-ski, Japan June 30, 1969 June 28, 197 1 Tokyo Shibaura Electric Co., Ltd. Kawasaki-slit, Japan July 5, 1968 Japan lnventors App]. No. Filed Patented Assignee Priority COATED METAL MOLD 3 Claims, No Drawings References Cited UNITED STATES PATENTS Primary Examiner-J. Spencer Overholser Assistant Examiner-John E. Roethel Attorney-Kemon, Palmer & Estabrook ABSTRACT: A metal mold having a mold cavity for casting metals therein, the wall of which comprises a substrate made of cast iron or mild steel, an intermediate layer about 30 to about 100 microns thick formed on the surface of said sub strate and containing about 6 to about 12 percent by weight of silicon diffused therein and a layer to nickel-aluminum or nickel-chromium alloy deposited on the surface of said intermediate layer by means of plasma spraying or flame spraying.

column METAL MOLD FIELD OF THE INVENTION The present invention relates to a metal mold having a mold cavity for producing castings by pouring a liquid metal into it and more particularly to a metal mold having the inner surface of said mold cavity provided with durable coatings.

BACKGROUND OF THE INVENTION A metal mold of the type used in casting metals is demanded to be chemically and physically stable to a liquid metal to be cast at the temperature of said both. To meet such demand, there has been proposed a metal mold whose inner wall surface is coated or chemically bonded with a layer of chemically and physically stable material, such as refractory oxides or pyrolytic graphite. However, the main problem encountered with a metal mold protected by such coating is that when a liquid metal is poured into the mold the coating tends to come off the substrate due to the thermal shocks applied to the mold, because the coating and substrate have different coefficients of thermal expansion and there is formed during operation a layer of oxides in the interface between the substrate and coating. The present invention provides an effective approach to resolution of such problem.

SUMMARY OF THE INVENTION Generally, the present invention provides a metal mold, the wall of which comprises a substrate made of cast iron or mild steel, a siliconized intermediate layer about 30 to about 100 microns thick formed on the surface of said substrate and a top layer of nickel-aluminum or nickel-chromium alloy deposited by fusion bonding such as plasma spraying or flame spraying on the surface of said siliconized intermediate layer. Said siliconized intermediate layer contains about 6 to about 12 percent by weight of silicon so as to display satisfactory antioxidation properties under the casting conditions and be prevented from exhibiting a noticeably different coefficient of thermal expansion from that of the substrate. That plane of said siliconized intermediate layer which contacts the top layer formed thereon has an extremely great resistance to oxidation, effectively protecting the top layer from the occurrence of cracks or preventing it from coming off. Accordingly, the metal mold of the present invention can withstand a far larger number of casting cycles than the prior art coated metal mold.

DETAILED DESCRIPTION OF THE INVENTION As mentioned above, the wall of the metal mold according to the present invention comprises a substrate made of cast iron or mild steel, a siliconized intermediate layer and a top layer. The siliconized intermediate layer may be prepared by processing the desired surface of the substrate itself using customary practice instead of separately providing it. For example, it is possible to heat the desired surface of the substrate to 950 to l,000 C in a state contacting an atmosphere consisting of siiicon tetrachloride and hydrogen, thereby forming the aforesaid siliconized layer containing about 6 to about 12 percent by weight of diffused silicon. An alternative process is to allow silicon-containing powders such as Si, Fe-Si or SiC to contact the surface of the substrate, and heat the mass to 1,000 to l,l50 C in an atmosphere of chlorine gas streams, thereby forming a siliconized layer containing a desired proportion of diffused silicon.Silicon-containing powders used in siliconization preferably consist of Fe-Si-Cr alloy. This alloy can be prepared in extremely high purity, for example, by vacuum melting and protected from the harmful effect of oxygen occurring as when there are present oxides, thus permitting the formation of'a more compact siliconized layer. Where there are employed powders of the aforementioned alloy, the resultant siliconized layer will contain small amounts of chromium. It will be understood, however, that the object of the present invention will not be adversely affected at all by the inclusion of chromium.

Experiments show that the aforesaid siliconized layer should have a thickness of at least about 30 to about 100 microns. A siliconized layer having a thickness falling within this range can be prepared at a commercially feasible speed and exhibit sufficient resistance to oxidation. It is further disclosed that said siliconized layer should contain about 6 to about 12 percent by weight of diffused silicon in order to be effectively resistant to oxidation and have a coefficient of thermal expansion which does not widely vary from that of the substrate.

The thickness of the siliconized layer mainly depends on the time and temperature associated with siliconization. The higher the temperature and the longer the time used in siliconization, the thicker will be the resultant siliconized layer. Also where siliconization is carried out using powders of silicon-containing alloy, the thickness of the resultant siliconized layer varies with the amount of Si contained in said alloy. There were prepared several siliconized layers from powders of Fe-Si alloys containing different proportions of Si, with the processing temperature fixed at l,100C and processing time The amount of silicon contained in the siliconized layer is little affected by the conditions in which said layer is prepared, but remains substantially fixed with respect to thesiliconized method employed. Accordingly, where the siliconized layer is desired to contain larger amounts of difi'used silicon, the same or different siliconizing methods are repeatedly employed until the content of said diffused silicon reaches a desired level. However, theincorporation of about 6 to about 12 percent by weight of diffused silicon as intended by the present invention will be attained by two cycles at most of the aforesaid or other known suitable siliconizing processes.

The top layer disposed on the inner wall surface of a metal mold according to the present invention, that is, the layer which comes in contact with a liquid metal poured into said mold is prepared by depositing by plasma spraying on flame spraying a layer of nickel-aluminum or nickel-chromium alloy on the surface of the intermediate layer. Formation of said top layer can be effected by depositing either of the aforesaid alloys using the known plasma spraying or flame spraying apparatus. The Ni-Al or Ni-Cr alloy to be used may consist of those marketed by Metco Inc., for example, Powder 0404 (Ni-Al) or Powder 043C (Ni-Cr). While the thickness of the top layer is not subject to any particular limitation, the most preferable thickness will range from about 0.l mm. to about 0.5 mm.

There were conducted a series of tests to confirm the durability of a metal mold according to the present invention. The inner wall of the tested metal mold comprised a substrate made of cast iron, an intermediate layer about 50 microns thick formed on the surface of the substrate and containing about 10 percent by weight of diffused silicon and a top layer about 0.2 mm. thick formed on the surface of the intermediate layer. There were prepared two metal molds, in which the top layers consisted of percent Ni20 percent Al alloy 80 percent Ni-20 percent Cr alloy respectively. Siliconizing the intermediate layer was carried out by embedding a material to be processed in a mixture of 66 percent ferrosilicon powders containing about 25 percent silicon, 30 percent alumina powders and 4 percent NI-LCI powders and holding the mass at about l,lO0C for about 4 hours in an atmosphere of hydrogen. Each of the tested metal molds was subjected to a number of cycles of allowing its interior to be maintained at a temperature of l,00OC in the air for 15 minutes and quenching it with water to 22C. Seven cycles of said heating and quenching cycles did not produce any noticeable change in the top layer. It was only in the eighth cycle that the top layer displayed a small number of fine cracks.

By way of comparison, there were prepared two other metal molds by depositing top layers similarly composed of Ni-Al and Ni-Cr alloys respectively directly on the surface of the substrate, namely, without using the aforementioned siliconized intermediate layer. When both molds were tested under the same conditions as described above, the top layer thereof came off at several points only after one or two cycles of heating and quenching.

Although the conditions of the aforesaid test were much severer than those to which a metal mold is normally subjected, the metal mold of the present invention displayed, as clearly seen form the test results, far more excellent resistance to thermal shocks than the ordinary type. While the cause for such prominent durability of the present metal mold is not fully understood, one of the important reasons seems to be that the plane of the intermediate layer which faces the top layer has very great resistance to oxidation. In fact, the microscopic examination of the interface between the top layer and intermediate layer of the first group of tested metal molds and between the top layer and substrate of the second group of tested metal molds shows that while the first group did not present any change in the interface, the latter group displayed the formation of oxides and occurrence of gaps in the interface.

There were further performed a series of experiments to make sure of the oxidation resistance of the siliconized intermediate layer. For these experiments, there was prepared a metal mold, the inner wall of which comprised a substrate made of cast iron, a siliconized intermediate layer and a top layer. Said siliconized intermediate layer was prepared under the same conditions in which the similar intermediate layer of a sample metal mold used in the first mentioned test was formed. There was prepared another metal mold of substan- TABLE II Sample Sample metal metal mold mold 3 Heating time, hr.

l Lacking a siliconized intermediate layer. 9 Having a siliconized intermediate layer tially the same type excepting that it lacked a siliconized intermediate layer. These two sample molds were heated to 900 C in the air and their weights were measured after each specified length of time. Table 11 below presents changes in weight mg. per 1 square centimeter for the duration of heating applied.

metal mold having a siliconized intermediate layer first increased afier 20 hours, but later decreased. This was due to the combustion of the graphite contained in the substrate. Microscopic examination shows that the interface between the top layer and intermediate layer presented substantially no formation of iron oxides. In contrast, the other sample metal mold which was not provided with a siliconized intermediate layer has already distinctly indicated the formation of iron oxides after 30 hours of heating. Obviously, the appearance of a layer of said iron oxides originated with the fact that oxygen gas penetrated through the porous top layer formed by plasma spraying or flame spraying up to the surface of the substrate to cause it to be oxidized.

Accordingly, it will be apparent that as mentioned above, the metal mold of the present invention prevents the top layer formed by plasma or flame spraying from coming off the substrate due to the interposition of a siliconized intermediate layer, and in consequence is saved from the shortening of its.

We claim: 1. A metal mold having a mold cavity for casting metals therein, the wall of which comprises a substrate made of iron material selected from the group consisting of cast iron and mild steel, a siliconized intermediate layer about 30 to about' 100 microns thick formed on the surface of the substrate and containing about 6 to about 12 percent by weight of diffused silicon and a top layer made of an alloy selected from the group consisting of nickel-aluminum alloy and nickel-chromium alloy and deposition on the surface of the intermediate layer by fusion bonding.

2. A metal model according to claim 1 wherein the top layer formed by fusion bonding consists of an alloy of percent nickel and 20 percent aluminum.

3. A metal mold according to claim 1 wherein the top layer formed by fusion bonding consists of an alloy of 80 percent nickel and 20 percent chromium. 

